This invention relates to apparatus for generating or detecting magnetic field components in a magnetic resonance system. More particularly, the invention relates to improved apparatus suitable for use in a magnetic resonance system for generating or detecting magnetic field components transverse to a line or axis along which a static magnetic field is directed. The term "magnetic resonance system" refers to a system that employs the phenomenon known as magnetic resonance, for example, nuclear magnetic resonance (NMR) typically to determine characteristics of materials placed within the system. The system generates magnetic resonance in the materials by exposure of them to a radio-frequency magnetic field having components transverse to the line or axis along which a static magnetic field is directed.
Magnetic resonance systems generally have employed either solenoid or saddle-shaped radio-frequency coils. The coils are used to generate and/or to detect magnetic field components oscillating in directions transverse to the direction of the static magnetic field.
Magnetic resonance systems have been used for many years in spectroscopic applications for analysis of the atomic or molecular structure or materials placed in the system for examination. When radio-frequency pulses are supplied to an electrical coil surrounding the specimen to be examined, the radio-frequency magnetic-field component transverse to the axis or direction of the static magnetic field can cause the net nuclear magnetic field of the specimen to change in orientation. In the absence of the RF field, the nuclei of the specimen together have a net magnetic field that results from, and that is in alignment with, the static magnetic field in which the specimen is placed. When the RF pulse occurs, the magnetic-field components transverse to the direction of the static magnetic field cause the net magnetic field to change its orientation. The direction of the net magnetic field of the specimen being examined then precesses about the line or axis in the direction of the static magnetic field. This precessing, or time-dependent magnetic field as a result of Faraday's law induces an EMF in an electrical coil or antenna surrounding the specimen. The frequency of precession and of the induced EMF is called the Larmor frequency, is proportional to the strength of the magnetic field to which the specimen is exposed at the time of the precession, and is dependent upon the structure of the nuclei, as well as upon the environment in which the various nuclei are found. Thus, precession of the net magnetic field is magnetic resonance. The frequency of resonance depends upon nuclear structural properties, sometimes called "spin" properties.
The structure or "spin" properties of the nuclei of the various elements that constitute matter vary considerably. Most elements evidence little in the way of detectable magnetic resonance characteristics. Hydrogen produces the strongest NMR response, but other elements, such as phosphorus 31 or carbon 13, also exhibit strong magnetic-resonance responses to RF stimulation in the presence of a magnetic field.
The magnetic resonance phenomenon recently has found application in medical diagnostic-imaging systems. These systems are intended to produce images of the internal structure of the human body. To obtain images of high quality, it is essential that the radio-frequency magnetic field to which the human anatomy is exposed be spatially as uniform as possible in directions transverse to the static magnetic field.
The present invention provides apparatus for generating oscillatory magnetic-field components transverse to the direction of a main static magnetic field. The oscillatory magnetic fields can be, at an RF frequency, spatially uniform while still accommodating the needs of magnetic resonance as applied to imaging of human body structure. Prior art radio-frequency generating and detecting apparatus used in magnetic-resonance imaging systems perhaps is best described in the recent book by Kaufman, Crooks, and Margulis entitled "Nuclear Magnetic Resonance Imaging in Medicine", Igaku-Shoin, New York, 1981, particularly pages 53 through 67 relating to hardware used in NMR imaging.